In writing on a semiconductor wafer with an electron beam or ion beam for microfabrication or resist exposure, the scanning range of the beam is quite often insufficient to scan over the entire desired area of the wafer. Also, multiple exposure levels on the same wafer must be accurately overlaid with each other. As a consequence of this, the wafer is mounted upon a positionable stage movable on two orthogonal axes. After one scan pattern is completed, the stage moves the wafer on one or both axes to a new position for writing of another scan pattern. This second scan pattern must be related in position to the first one. To achieve the coordination, topographic benchmarks are positioned with respect to the first scan pattern. Thereupon the stage moves the wafer so that it can be written with the second scan pattern. The position of the benchmark is detected and controls the stage for proper positioning of the wafer on the stage for writing the second scan pattern. For multiple level exposures, the same benchmarks are used to position the wafer for each exposure level. The benchmarks may be placed on the wafer initially in a separate operation (either optically or with an electron or ion beam), or they may be incorporated in the first pattern layer to be exposed on the wafer.
U.S. Pat. Nos. 3,849,659, 4,056,730 and 4,145,597 use four detectors for finding the orthogonal position of a benchmark. U.S. Pat. Nos. 4,119,854, 4,123,661 and 4,199,688 use single benchmark detectors. In addition, Donald E. Davis in two articles in the IBM Journal of Research and Development, one in November 1977, page 498-505 and the other at Vol. 24, No. 5, September 1980, pages 545-553 teaches the use of four orthogonally positioned detectors for determining benchmark position. The signals from four detectors are necessarily complex to determine the benchmark location, and the use of a single detector provides inadequate resolution.